Editing Ylide (section)

==Phosphonium ylides==
:[[Image:Ph3P=CH2-from-xtal-1989-3D-balls.png|thumb|right|240px|Structure of [[methylenetriphenylphosphorane]]]]

Phosphonium ylides are used in the [[Wittig reaction]], a method used to convert [[ketones]] and especially [[aldehyde]]s to alkenes. The positive charge in these [[Wittig reaction#Wittig reagents|Wittig reagents]] is carried by a [[phosphorus]] atom with three [[phenyl]] substituents and a bond to a [[carbanion]].  Ylides can be 'stabilised' or 'non-stabilised'. A phosphonium ylide can be prepared rather straightforwardly. Typically, [[triphenylphosphine]] is allowed to react with an [[alkyl halide]] in a mechanism analogous to that of an [[SN2 reaction|S<sub>N</sub>2 reaction]]. This [[quaternization]] forms an alkyltriphenyl[[phosphonium]] salt, which can be isolated or treated in situ with a strong base (in this case, [[butyllithium]]) to form the ylide.
:[[File:Ph3PCH2prep.png|440px]]

Due to the S<sub>N</sub>2 mechanism, a less sterically hindered alkyl halide reacts more favorably with triphenylphosphine than an alkyl halide with significant steric hindrance (such as [[tert-butyl bromide]]). Because of this, there will typically be one synthetic route in a synthesis involving such compounds that is more favorable than another.

Phosphorus ylides are important reagents in organic chemistry, especially in the synthesis of naturally occurring products with
biological and pharmacological activities. Much of the interest in the coordination properties of a-keto stabilized phosphorus
ylides stems from their coordination versatility due to the presence of different functional groups in their molecular structure.